Manufacture of a laminated plate begins with stacking a laminated plate material to prepare an object that will be compressed. After being delivered between hot platens, the object is applied with heat and pressure to fluidize and cure thermoplastic resin or thermosetting resin contained in the laminated plate material to make the material into one piece.
If the hot platens and the delivered object have uneven thickness, thermal strains, temperature nonuniformity and so on in the hot pressing process, uniform temperature and pressure cannot be applied to the object, which will later become a product. As a result, the final product may have defects such as thickness defects, cracks and warpage. A preventive measure against such thickness defects of the final product is the use of a cushioning member during press forming. The materials used for the cushioning member include paper, such as kraft paper and linter pulp paper, woven fabric, needle-punch non-woven fabric, rubber, laminated plates of woven fabric or non-woven fabric and rubber, and so on.
In recent years, laminated plates including printed circuit boards tend to have more precise circuits, a greater number of layers and higher heat resistance. Therefore, the laminated plates are required to have an exact thickness. In terms of manufacture of the laminated plates, the use of such high-heat-resistance materials induces the press-forming temperature to be increased more and more. In addition, for the purpose of improving production efficiency, formed products tend to increase in size, and temperature cycle between heating and cooling tends to be accelerated. In such circumstances, the above-described conventional cushioning members are inadequate to deal with the following problems.
The first problem is degradation of thickness accuracy in a plane of the laminated plate. The degradation of thickness accuracy means that the final product has a thickness gradually decreasing from its center toward its end and results in appearing like a convex lens. Especially, around the thinner end part, product deficiency, like thin spots, is developed. The degradation is caused by outward flow of resin in the end part of the laminated plate material, which makes the end part thinner.
The next problem is defects in multi-layered laminated plates (MLB) or other laminated plates provided with inner-layer circuits. The MLB (multi-layered laminated boards) has areas, where the inner-layer circuits are not formed, which are recessed. The recessed areas are likely to develop defects such as voids and thin spots and creases of copper films.
The other problem is residual strains and warpage in products caused by the curing contraction and thermal contraction of resin occurring during manufacture of the laminated plate. Especially, in the case of a laminated plate made of a thermosetting resin, the residual strains in the laminated plate are likely to cause cracks, so-called measling, in the printed circuit board during a manufacturing process such as a drilling and punching process.
In order to handle these problems, Japanese Unexamined Patent Application Publication No. 6-126855 discloses a cushioning member with a film that fluidizes during hot pressing to improve the thickness accuracy of laminated plates and prevent measling from occurring. Japanese Unexamined Patent Application Publication No. 9.29773 uses rubber as a cushioning member to prevent the measling from occurring; the rubber exhibits a specific storage elastic modulus.
However, the conventional cushioning members are inadequate to meet the recent performance demand.